To determine the conversion factor (CF) of the sural nerve the correlation between the maximum nerve conduction velocity (NCV) and the diameter of the largest fibers was studied in 30 patients suspected of having neuropathy. Sensory nerve action potentials were obtained by on-nerve needle nerve conduction study using needle electrodes placed on the exposed sural nerve during biopsy. The CF was 4.3 (n = 2) in normal sural nerves and close to the normal value (3.85, n = 4) in axonal neuropathy. The CF in demyelinating neuropathy was smaller than the normal value (2.77, n = 24), indicating disproportionately slower conduction than expected from the diameter of nerve fibers. The CF was helpful in differentiating between demyelinating and axonal neuropathies. We propose that a 36% decrease from the mean value of NCV is a reasonable criterion for demyelination of the nerve. © 2010 Wiley Periodicals, Inc.